Galvanometer Detects small currents Diode Allows current in one direction only Inductor Stores energy in a magnetic field Capacitor Stores energy in an electric field

Electric Potential and Potential Difference

Electric Potential (V): The work done per unit positive charge by an external agent in moving the charge from a reference point (usually infinity) to a specific point in an electric field without acceleration.

$V = \frac{W_{\text{infinity} \to P}}{Q}$

Potential Difference ($\Delta V$ or pd): The work done per unit positive charge to move a charge from one point to another in an electric field without acceleration.

$\Delta V = V_B - V_A = \frac{W_{AB}}{Q}$

Where $W$ is work done (in Joules, J) and $Q$ is charge (in Coulombs, C).
SI Unit: Volt (V). 1 Volt = 1 Joule/Coulomb.
Measured by a Voltmeter, always connected in parallel across the points where potential difference is to be measured.
An ideal voltmeter has infinite resistance.
Electromotive Force (EMF, $\mathcal{E}$): The potential difference provided by a source (like a battery) when no current is flowing through it (open circuit). It's the maximum potential difference the source can provide, representing the energy provided per unit charge.

Ohm's Law

States that the current ($I$) flowing through a conductor between two points is directly proportional to the potential difference ($V$) across the two points, provided the temperature and other physical conditions remain constant.

$V \propto I \implies V = IR$

Where $R$ is the constant of proportionality called Resistance.
Ohmic Conductors: Materials that obey Ohm's Law (e.g., most metals at constant temperature). Their V-I graph is a straight line passing through the origin.
Non-Ohmic Conductors: Materials that do not obey Ohm's Law (e.g., semiconductors, diodes, transistors). Their V-I graph is non-linear, a